1. Field of the Invention
The present invention relates to an apparatus and a method for adaptivety controlling an array antenna, and in particular, to an apparatus and a method for adaptively controlling an array antenna composed of a plurality of antenna elements, comprising an adaptive control means with an improved initial value setting arrangement.
2. Description of the Related Art
In order to establish higher communication quality in mobile communication, it is required to provide a function of always capturing a desired wave as well as another function of removing frequency-selective fading occurring in a multi-path propagation. For the latter function, it is known to those skilled in the art that, for example, the constant modulus algorithm (referred to as the "CM algorithm" hereinafter) is effective in removing an unnecessary wave which is a delayed wave having correlation with a desired wave (See, for example, Ohkane et al., "Characteristics of CMA Adaptive Array for Selective Fading Compensation in Digital Land Mobile Radio Communications", The Institute of Electronics Information and Communication Engineers in Japan, Transactions, Vol. J73-B-II, No. 10, pp 489-497, October in 1990 (referred to as Reference 1 hereinafter).)
Prior to the processing according to the CM algorithm, the following beam forming process and beam selecting process which are known to those skilled in the art are executed. 10 (a) Beam forming process: a plurality of N beam electric field strength E.sub.n (an electric field strength is referred to as a field strength hereinafter) are calculated based on a plurality of M reception signals received by respective antenna elements of an array antenna, directions of respective main beam of a predetermined plurality of N beams to be formed which have been previously determined so that a desired wave can be received in a predetermined range of radiation angle, and a reception frequency fr of the reception signals.
(b) Beam selecting process: By comparing the above-mentioned plurality of N beam field strengths calculated in the beam forming process with a predetermined threshold value, only beam field strengths greater than the threshold value is selected and then outputted.
According to the above-mentioned CM algorithm, based on the plurality of N or less beam field strengths selected by the beam selecting process, there are calculated a plurality of N weight coefficients w.sub.n (n=1, 2, . . . , N) for the reception signal corresponding to respective beams, so that the main beam of the array antenna is directed toward a desired direction of a desired wave and also the received signal levels in arrival directions of unnecessary waves such as interference waves or the like become zero. In other words, the CM algorithm is to make the received signal level in the radiation pattern of the array antenna in the arrival directions of the unnecessary waves such as interference waves or the like by converting a waveform of an envelope changing due to an influence of the unnecessary waves into a desired waveform in a communication system using a signal of the desired wave whose envelope is known, as described in detail hereinafter.
In a conventional array antenna using an adaptive control algorithm such as the above-mentioned CM algorithm or the like, the influence of the delayed wave can be removed by adaptively controlling the directivity of the antenna, however, the delayed wave is merely removed and is not utilized. In order to give solution to the above-mentioned problem, a method for diversity-receiving signals with separating a direct wave and a delayed wave is disclosed, for example, in Kuroiwa et al., "Design of a Directional Diversity Receiver Using an Adaptive Array Antenna", The Institute of Electronics Information Communication Engineers in Japan, Transactions, Vol. J73-B-II, No. 11, pp 755-763, November in 1990) (referred to as a "conventional example" hereinafter.)
In the conventional example, the diversity reception is achieved by separating a direct wave and a delayed wave from signals received in the following procedure.
(a) Only the direct wave is taken out according to the conventional adaptive control algorithm.
(b) Then an adaptive equalizer is made to operate using the direct wave thus taken out as a reference signal to take out only the delayed wave.
(c) Finally, the diversity reception is achieved by multiplying the direct wave and the delayed wave, which have been thus taken out, respectively, by weight coefficients, so as to obtain the maximum signal-to-noise ratio.
However, the conventional example has the following problems.
(a) The conventional adaptive control algorithm is used and the adaptive equalizer is made to operate after satisfying a predetermined convergence condition in the process according to the above-mentioned algorithm, and this results in relatively increase in the time required for the adaptive control process.
(b) It is required to provide different processing units of, for example, a CMA processor and the adaptive equalizer, and then this results in a complicated hardware structure.